The imperative Get this generally introduces a statement that is expected to be surprising or impressive in some way. In this particular context, it means approximately pay attention, because what I am about to tell you is amazing in a good way!
It's an informal phrase; I would most expect to see it in dialogue or actual speech, either in a situation where the speaker is laying out truly surprising or unexpected facts or as mild hyperbole. In fiction, it seems to pop up particularly in mysteries or from the mouths of young people. I believe the latter is the context in the example.
Possible substitute phrases might be check this out or can you believe it?
Some other examples1 of usage (bolding added):
‘But get this,’ continued his companion, their voices fading as they moved deeper into the building, ‘she says her mother hates Americans because one of them near raped her in the war.’
Get this. Where had Rebus heard that expression before? He fumbled in his jacket pocket and found a folded piece of paper. Unfolded it and began to read.
‘GET THIS, I'M NOT HOMOSEXUL, O.K.?’ It was the photocopy of the
Wolfman's letter to Lisa.
Get this. It did have a transatlantic ring to it, didn't it? A curious way altogether of starting a letter. Get this. Be warned,
watch out. There were several ways of starting a letter so that the
reader knew he was to pay particular attention to it. But get this?
(Ian Rankin, Tooth and Nail, 2009. Full quote available in The Complete Rebus Collection.) Note that the novel is set in the
UK, so transatlantic here means (North) American; this also suggests that the phrase isn't common in British English.
"All four of them have been guests of various government facilities,
mostly violent crimes. Assaults, assaults with deadlies.
Spine-crackers and persuaders, from the sheets. But get this."
She lowered her voice even more, so that McNab had to lean in, catch a
teasing whiff of her shampoo. "They're connected to Max Ricker."
(J.D. Robb/Nora Roberts, Judgment in Death, 2000.)
"So Sam just called me. Like, he just woke up. In Todd's loft. On the
couch. But get this, he woke up totally nude, with his leather
jacket tied around his ankles."
(Nina-Marie Gardner, I'm Not This Girl, 2012)
Mr. BYRD. . . . Let me read a section of the Constitution to Senators.
Section 7 of article 1, Paragraph 1:
All Bills for raising Revenue shall originate in the House of
but the Senate may propose or concur with Amendments as on other
Now, we all know that when fast track is brought to the Senate,
Senators may not propose amendments. In my way of reading the
Constitution, that is not in accordance with what the Constitution
(Congressional Record, V. 147, PT. 18, December 11, 2001 to December 12, 2001) It doesn't get much more formal than a Congressional hearing, so apparently this phrase does get used in formal speech. But I still wouldn't recommend it for formal writing.
1 Most of these examples are in the form "but get this" because that construction is easier to search for; I suspect the plain form "get this" is also very common, but it gets swallowed up in searches by phrases like "get this soon because it won't last" and "let's get this party started" and "get this thing off of me!"